1. Field of the Invention
The present invention relates generally to the trimming of optical fiber components and particularly to a method and apparatus for achieving the trimming of the optical path length of an optical fiber component.
2. Technical Background
Optical fiber based devices are widely utilized as components for optical communications due to their relatively low insertion loss and low cost. Foremost of optical fiber components are fiber Bragg gratings (FBG) which are typically made by ultraviolet (UV) wavelength energy exposure. Once an FBG is mounted to a substrate and annealed, it is no longer photosensitive and cannot be further tuned. Thus, it is necessary to empirically predict the final frequency of such a grating which can lead to a significant error and gratings which are not within specifications. Due to the uncertainty of the wavelength shift resulting from the attachment process and annealing, the center wavelength of a package fiber Bragg grating typically has an error of +/xe2x88x9220 picometers from the desired center wavelength. Such a wavelength error combined with a wavelength drift of, for example, distributed feedback lasers, which may be from +/xe2x88x9250 picometers, and the residual temperature dependence of +/xe2x88x9220 picometers imposes a highly stringent requirement on the design of, for example, 50 GHz fiber Bragg gratings.
Infused fiber Mach-Zehnder interferometers are also wavelength selective and are used in a variety of communication devices, such as optical switches, filters, wave division multiplexers, demultiplexers, and add/drop filters as examples. In Mach-Zehnder based devices, the optical performance critically depends on the phase difference and/or optical path length difference between two interfering arms. Phase trimming has been attempted utilizing UV exposure to the fibers, however, such fibers must be photosensitive and, once annealed after such UV exposure, the trimming processes cannot be further controlled. Additionally, the maximum amount of trimming utilizing UV exposure is limited to a few wavelengths due to the relatively small refractive index change induced by UV radiation. In some applications, such a trimming process may not be sufficient to achieve the optical path length change necessary.
With optical path length sensitive fiber-based devices, therefore, not only is the tuning range a serious limitation by prior techniques, so is the tuning accuracy. There exists a need, therefore, for a system for the tuning of fiber optic devices over a relatively wide band of wavelengths, as well as to a precise wavelength.
The method and apparatus of the present invention achieves the tuning of fiber optic devices by, in one embodiment, precisely heating a small area of a fiber adjacent a grating mounted under tension to allow the grating length to change. By pulsing a source of heat in controlled amounts, the optical length of such a grating can be precisely controlled within 1 picometer precision over a tuning range of about 200 picometers.
In fibers having dopants which can be diffused, the length of an optical fiber can be trimmed with nanometer precision by thermal diffusion which directly affects the refractive index of the fiber, thereby effectively changing its optical path length.
In either embodiment, real time tuning is achieved by injecting a broad band source of energy at the input of the device and coupling a spectral analyzer at its output to monitor the center frequency of the optical device during trimming using a controlled source of localized energy applied to the optical fiber. In a preferred embodiment of the invention, the energy source comprised a laser and particularly a CO2 laser having a relatively narrow beam corresponding to the diameter of the optical fiber employed in the device. Energy from the laser is directed to a small area of the optical device in pulses which provide precise control of the trimming process.
A method of trimming an optical fiber component by directing a source of radiation onto a section of the component for heating the section, coupling a broad band source of signals to an input of the component, coupling an optical analyzer to an output of the component, and monitoring the signal at the output of the component while selectively applying the radiation to the component from the source to achieve a predetermined trimming effect.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description are exemplary of the invention only and are intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provided a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.